Previously, the documentation only provided examples of the required
configuration for Maven users. This commit adds equivalent configuration
snippets for those using Gradle. It also removes the recommendation to
override the version of the Servlet API as this is unnecessary. The pom
files for the Jetty 8 and Tomcat 7 samples have also been updated
accordingly.
Closes gh-2346
Switch the Tomcat 8 and Jetty 9 samples to Tomcat 7 and Jetty 8. Since
Tomcat 8 and Jetty 9 are now the default versions the previous samples
are no longer relevant, however, we do need integration tests to check
that we remain compatible with Tomcat 7 and Jetty 8.
See gh-1832
